Amplifying circuits



Aug. 20, 1929.

AMPLIFYING CIRCUITS Filed May 11, 1926 F. X. RETTENMEYER HHIHHMH Patented Aug. 20, 1929.

UNITED STATES PATENT OFFICE.

FRANCIS X. RETTENLEYER, OF MONTCLAIB, NEW JERSEY, ASSIGNOR T0, 'WESTERN ELECTRIC COMAPNY, INCORPORATED, 0F NEW YORK, N. Y., A CORPORATION OI' NEW YORK.

AMPLIFYING CIRCUITS.

Application filed lay 11,

This invention relates to amplifying systems and more particularly to energizing circuits i'or space discharge amplifiers.

An object of the invention is the suppression in the output circuits of space discharge devices, such as amplifiers, of noise currents due to voltage ripples that may be unavoidably present in the continuous voltage of the energizing source.

To accomplish the suppression of the noise` currents use is made of a circuit arrangement similar in certain respects to that disclosed in U. S. Patent No.'1,541,311, issued June 9, 1925, to S. E. Anderson, but which is an im` provement thereover, whereby smaller and less expensive impedance elements may be used to secure the desired result.

An additional feature of the invention permits both the filament and the space path of an amplifier to'be energized from the same source, a single compensating arrangement serving to su press the noise currents due both to the effldctof the ripple voltage in the space circuit, and to the current in the filament.

These and other features ofthe invention are more clearly set forth in the following detailed description of the inventionand of its ,mode of operation. In the drawing, which forms part of the descriptiom,

presence of the ripple Fig. 1 shows in' schematic'jform a simple amplifier circuit illustrating the im roved arrangement for neutralizing the e ects ofv a noise producing ripple; and

Fig. 2 shows in. more complete-schematic form a detecting and amplifying stem embodying the several features of the invention.

In Fig. 1 a three electrode space discharge .device 1 is furnished with space current from a power source 2, which may be an A. C.

rectifier', or a direct 'current generator orl other source of direct current, the voltage of which is subject to more or -less periodic fluctuations.

The circuit is adapted for amplifying signal waves which are received by input transformer 3, and which, after amplification, are transmitted by output transformer 4. Across 1926. Serial No. 108,229.

the terminals of the source 2 are connected in series two impedance'networks, one of which comprises a parallel combination of resistance the cathode is connected to the junction point Y of the two networks. i

Current from source 2 divides between the space path and the path containing network RljCl, the two currents combining again at Y the junction point of the two networks and the cathode, and flowing together through network R3 R4 C3.

It is necessary that a steady space current should flow in the space path of the discharge device. It is therefore desirable that the rip-V ple component be suppressed in that path to prevent it appearing as a source of noise 1n the output circuit of the discharge device.

The suppression of the ripple current in the space path is effected by impressing oppositelv phased ripple E. M. F.s upon the grid and the plate of the discharge device, the'relative values of the opposing E. M. F. being determined by the amplification constant of the device. The potential impressed upon the plate of the discharge device from source 2 is Adetermined largely by the impedanee'iof network IR1 C1 and the internal resistance of the discharge device, which will be denoted by R, while the potential impressed upon the grid is equal tothe fall of potential in capacity C, due tothe current traversing the network R3 IR4 0,.

If the total current, including both the steady current and the ripple com onent, delivered by source 2 be denoted by thenthe E. M. F..denoted by Ec, impressed upon the capacity were included in the grid circuit to grid, is given by the formula R3 Z3 R3 Ril-l- Za R3 Rad-R4 R3+R4+ Z3 in which Z3 is the impedance of capacity C3. The factor is a simple numerical. quantity, which may v -be denotedl by n, and the factor- The fraction of the total current Iothat flows .through the space path may be denoted by I2 and is found' to be in accordance with the equation in which Zr is the load impedance introduced into the circuit by transformer 4; Z1 is the impedance of capacity C1, and ,i is the amplication constant of the vacuum tube.

Since the terms ZIRI Z1 -l- R1 and lboth represent impedances of the same type it is possible to construct the network so that these terms are equal at all frequencies, in which case no current flows in the space path.

Considerations of transmissionof the amplified signal waves require that a lowim- If a simple combination of resistance and balance the impedance of the C1 R1 combination it would require a capacity p. times as large as C1, and with many types of vacuum tubes the value of the grid circuit capacity might be as high as 6 to 12 microfarads, or even greater.

By using the arrangement of the invention, however, it is possible-to balance the capacity C1 by means of a relatively small capacity equal to n X ,a times C1, n being the ratio which is less than unity, and which may be made as small as desired by the proper choice of resistances R3 and R4.

The establishment of an exact balance between the E. M. F. impressed upon the grid and the plate from source 2 has the desirable effect of eliminating the ripple component from the space current, but it also has the effect of reducing the steady space current to zero. It is therefore necessary to upset the balance to some extent in order that a steady space current may flow. This may be done by inserting a low voltage battery 5 in the grid circuit, the battery being poled to impress a positive E. M. F. upon the grid, or, alternatively, it may be accomplished by increasing the resistance R1 to a value'greater than that required for a complete balance.

The irst method of upsetting the balance does not result in the introduction of any noise producing current in the space path, and, for circuits in which complete suppression is necessary, this is the preferred method. A voltage of from 6 to 12 volts is usually all that is required, and as no current is drawn from the lbattery it may be composed of very small dry cells.

The second method results in the introduction of a small amount of ripple current in the space path, but it has been found in practice that the amount of noise resulting is negligibly small. The greatest amount of ripple, or noise current that can be introduced when the balance is upset by increasing resistance Rl corresponds to the case in which R1 is infinitely great, that is,4 when the resistance is left out of the circuit entirely. For this case, the ratio of the space current to the total current is found to be in which R10 is the value of R1 required for complete balance.

At zero frequency this ratio is unity, but it decreases very rapidly as the frequency increases, and at the ripple frequency it may hundred by a proper choice of constants.

of which are determined by Equation 5.

In any specific case the values of the impe: dance elements are best determined by trial using as a guide the following equations by which their relative values are expressed.

Capacity C3 being determined by considerations of transmission efficiency the value of C1 may be chosen at will, the principal considerations being those of size and cost. Eqization 4 then determines the ratio of R, to 3.

The total steady current delivered by source 2 is limited by resistance R, and the parallel combination of R, and'`the resistance of the vacuum tube. When a rectifier is used as the supply source the total current that the rectifier can deliver at the operating voltage of the system determines the summation value of the resistances R, and R3, the relative values If a grid battery is used to produce the Y steady current, resistances R1 andR2 shouldbe exactly in the ratio determinedbyEquation'. If the grid battery-is not used the value of R, should be made such that the fall of potential in it due to the total steady current is that required for the proper polarization of the amplifier grid.

The application of the foregoing' principles in a practical system and their extension to the suppression of noise effects due to ripples in the filament current will be described in connection with the system shown in Fig. o

ing vataum tubes of standard design will also be given. It is to be understood, however, that the invention is not limited to the iiis particular system disclosed, but only in accordance with the limitations set forth in the claims.l

In the circuit of Fig. 2, vacuum tulbe 1 isa detector receiving modulated waves through the input transformer 3, and delivering detected signal waves, corresponding to speech for example, through output transformer 4 Numerical values of the impedance co-` efhcicnts suitable for use 1n circuits employthe negative potential of the grid required for most efficient detection, isequal to 12 volts. i The space current of the detector is only a small fraction of a milliampere. The amplifier tube 6 is designed to operate at a plate potential of 350 volts, a grid potential of -15 volts, and to take a steady space current of 30 lnilliainperes. The' amplification constant of this tube is also 6, and the filament heating current is one ampere.

The filament of amplifier G is heated by alternating current supplied to it through a balanced low voltage secondary winding 12 of the power transformer 10. the mid-point of the filament heating winding being connected to ground.

Rectified current is supplied to the plate circuits of the two vacuum tubes through filter 13. `This current contains a steady component and ripple components, mainly of 120 cycles and 240 cycles, which are largely suppressed by the filter. Space current for amplifier 6 is taken directly from the output of the filter, the path followed by this current being through the primary of transformer 7,

. the space path of amplifier 6, grounded conductor 15, the'filament of detector 1, and resistances 16 and 17, to the anode of the rectifier. The space current of amplifierlG is utilized to heat the filament of detector. 1, and, in the numerical example chosen, furnishes one-half of the lrequired heating current. The additional current required supplied by the rectifier through resistanees 14 and 18, the space current for the rectifier beingA also furnished through resistance 14.

Resistances 19 and 20 connect the grids of. tubes 6 and 1 respectively to points in the power circuit at potentials suitable to give the requisite polarizing potentials. Capacities 22 and 24.- provide low impedance return paths for the output currents from the discharge tubes and capacities 21 and 23 are the balancing capacities associated with condensers 22 4and 24 respectively.

In thekdetector circuit the elements 18, 22,

Y 16, 2O and 21 correspond respectively'to resistance R1, capacity (1 resistances R, and R4, and capacity CR of the system of Fig. 1. The vacuum tube filament is included with resistance 16 as part of the grid polarizing resistance. potential are therefore included in the grid E. M. F. and consequently are compensated along with the plate potential variations.

In the amplifier circuit condensers 23 and 2li correspond to capacities C., and C, of Fig. 1, and resistance 19 to R4, while resistances 17 and 16, the grid polarizing resistances, correspond to R3. In the amplifier circuit the resistance corresponding to R, is constituted by the two resistances 18 and 14 in series.

` In the specific example, condensers 22 and 24 are each of 2 microfarad capacity, this being a conveniently available unit, and of Fluctuations of the filament sufficiently large capacity to provide a path4 for the speech waves of impedance less than Resistances 16 and 17, each carrying a current of 60 milliamperes. are proportioned to provide negative polarizing potentials of 'l2 and 15 volts on the grids of tubes 1 and 6 respectively. This fixes the value of resistance 16 at 200 ohms and oi resistance 17 at 50 ohms. Resistance 20 may'be fixed arbitrarily at such a value as will make the proper capacity ot condenser 21 a convenient value. In the circuit illustrated account must be taken ot the fact that the current in the grid network is twice that-in` the plate circuit network, consequently condenser 20 should have. twice the capacity determined in accord ance with equation 5. Appropriate co-related values for resistance 20 and condenser 21 are 10,000 ohms arid 0.5 mfd. respectively.

Resistances 18 and 14 are proportioned by' trial, their proper values being such as will permit the required iilament current to flow and at the same time provide the proper plate potential for detector 1. For the particular circuit illustrated values of 2000 ohms and 8500 ohms respectively are found to be suitable. By similar procedures the values ot resistance 19 and capacity 23 are' established at 10,000 ohms and 0.3 mfd. respectively.

The proportions of the impedances in the energizing circuit as determined in this way leave a certain amount of the ripple current owing in each plate circuit.- This is due in part to the unbalancing of the Vcircuit necessary to permit the iiow of the steady space currents and in part to the interlinking of the energizing circuits of the two vacuuml tubes. In addition there is also a certain amount of 120 cycle current introduced into the output current of ampliier' by the alternating current used to heat the filament..

In most cases it is possible to neutralize to some extent the residual noise currents that are present in the system when the impedances have the computed values. The poling of the windings of transformer 4 may be arran ed so that the unbalanced current in the-p ate circuit of the detector when repeated by ampliier 6 opposes the unbalan'ee current in the amplifier output circuit. In addition one or more of the energizing circuit impedances, for example condensers 21 and 23, may be made adjustable, whereby the unbalanced current in the circuit of the one vacuum tube may be increased or decreased to secure the greatest degree of over all neutralization.

What is claimed is:

1. Ina wave transmission system, a three electrode space discharge device, a source'of space current therefor having an undulatory E. M. F., impedance means for impressing the E. M. F. of said source upon the space discharge path of said device, and means for preventing the flow of undulatory current in the space path of the dischar e device, said means comprising an impe ance network which is connected between the cathode of the lspace discharge device and the negative terminal of the current source, and which is constituted by a resistance element having'in parallel therewith a resistance element in series with a condenser, and a connection from the control electrode of said device to the junction point of said condenser and said series resistance.

2. In combination, a three-electrode space discharge tube, a source ot rectified alternating current, and means for energizing said tube from said source, said means comprisingv a pair of impedance networks included in seriesl between the terminals of said source, one of said networks comprising a resistance and a capacity in parallel, and the other of said networks comprising a resistance in parallel with a series combination of a resistance and a capacity, and said energizing means comprising valso connections from the anode, the cathode, and the control electrode, respec-y tively, to the outer terminal of the first mentioned network, the junction point of the two networks, and the junction point between the elements of the capacity-resistance combination in the second mentioned network, the capacity in said second network being. included between the control electrode connection and the cathode. l

3. In combination in` awave translating system, a three-electrodespace discharge-tube, having an anode, a control electrode and a filamentary cathode, a source of. undulatory current, a network comprising a resistance and a ca acity in parallel, said network being includedp in a path betweensaid anode and said cathode, a second network comprising a capacit in parallel with a divided resistance, sai second network being included in a path between said control electrode and said cathode, and connections to the positive and 4negative terminals ofsaid source .respectively from the outer terminal of said rst network and from an intermediate point in saidy d1'- vided "resistance, said networks being connected together in series, andtheir impedances being so proportioned that the ratio of the potential drops therein due toy the current irom` said source is e ual to the amplification constantofthe disc arge tube.

4.. In a wave translating system a combination in accordance with claim 3 characterized in that the said divided resistance ineltdesthe lamentary cathode of said tube.

In witness whereof, I Ahereunto subscribe'` my name this 7th day of May A. D., 1926.

FRANCIS .x RETTENMEYER. 

